1. Field of the Invention
This invention relates to electrophotography, and, more particularly, it is concerned with a novel method and device for cleaning a photosensitive screen having a multitude of fine openings therein for use in forming an image by modulation of ion current.
2. Description of the Prior Arts
The conventional electrophotographic method is generally such that, after formation of an electrostatic latent image on a photosensitive body, the thus formed latent image is caused to closely contact other chargeable image recording material such as, for example, electrostatic recording paper, and so forth to transfer the latent image onto the recording material, and finally this recording material is developed to obtain a desired image.
In the above-described electrophotographic method, unlike a method, wherein a toner image on a photosensitive body is transferred onto a recording medium such as image transfer paper, etc., as there are carried out no development and cleaning in preparation for such development on the photosensitive body, the surface of the photosensitive body receives very little physical damage.
Other methods of forming an image without necessity for developing the photosensitive body are taught in U.S. Pat. No. 3,713,734, U.S. Pat. No. 3,680,954, and so forth.
In this new image forming method by electrophotography which is called "screen process", a photosensitive screen having a multitude of fine openings to serve as the photosensitive body (hereinafter referred to simply as "screen") is able to form an electrostatic latent image in the picture image form on a recording material by modulating ion currents from electric discharge rather than simultaneously contacting the recording material to the photosensitive body. That is, the screen is not subjected to any mechanical work such as close contact of the recording material thereto, the cleaning thereof to be carried out after development at every image forming operation, and so forth, hence the screen is durable for use over a long period of time. In case, however, the surface of the abovementioned screen is made of an insulative substance (inclusive of photoconductive substance) such as, for example, vitreous selenium, resin, etc., there inevitably occurs change on the surface state of the screen which has been exposed to a corona discharge for a long period of time with the result that is becomes impossible to maintain sufficiently good electric charge thereon. (This of course applies not only to the screen of the present invention, but also to the conventional photosensitive body for electrophotography).
As a result of research and investigation, the cause for this phenomenon has been found to be due to the fact that very small dust in the form of fine particles of from 0.1 to 50 microns in diameter and consisting of electrically conductive substances and oxides of silicon, which are floating in the air, are electrically charged by the corona discharge to adhere to the surface and the openings of the screen. By this adherence of the fine particles, the insulative portion on the surface of the screen reduces its image resistance with the consequence that the electrostatic contrast of the formed image becomes low, or, when the dust is of a large particle size, it clogs the openings of the screen to bring about undesirable phenomena for the image formation. Further continuation of such adherence of the dust and dust-like fine particles would make it impossible to form visible picture images suitable for practical use. Particularly, in the case of the screen as the photosensitive body, most of the screen is flexible or supple in view of the raw material used to produce the same, so that employment of the conventional cleaning device to such screen is problematical for the reasons to be mentioned hereinbelow. That is, contact of the cleaning device to the surface of the screen at fairly high pressure would disturb the electrostatic latent image formed on the screen, or the friction between the cleaning member and the screen surface would deform or damage the screen surface to bring about undesired effects such as a shortening of the life of the screen, and so on. Furthermore, the dust adhered onto the opening of the screen cannot be removed by the surface rubbing along. Moreover, in the screen capable of making retention copies, such dust adherence inevitably decreases its capability for retention copying.
In the case of the photosensitive body for the xerographic reproduction system, there is carried out cleaning of the photosensitive body after transfer of the toner-developed image onto an image transfer material so as to remove excessive amount of the toner remaining on the photosensitive body. At this cleaning operation, the abovementioned dust which is detrimental to the image reproduction is also eliminated from the surface of the photosensitive body along with the toner, so that there takes place no problem due to accumulation of the dust on the photosensitive body.